Method of fabricating a semiconductor device with a bump electrode

Abstract: A semiconductor device has a connection electrode (4) and protective film (7) formed of organic material and covering the connection electrode (4). An opening (8) is formed in the protective film (7) to expose the connection electrode (4). A natural oxide layer is etched by argon-based dry etching. The surface layer of the protective film (7) is altered to reduce the insulativity in the dry etching process. After a projection electrode (14) is formed on the connection electrode (4) later, the altered surface layer of the protective film (7) is removed by oxygen-based dry etching. As no altered surface layer remain on the protective film (7), an adverse affect such as inadequate insulation does not occur.

Description: The present invention relates to a method of fabricating a semiconductor device. There is a semiconductor device (IC chip) mounting scheme called a TAB (Tape Automated Bonding) system by which a semiconductor device is mounted on a TAB tape. In this case, bump electrodes provided on a semiconductor device are connected to finger leads (inner leads) on the TAB tape by gold-tin eutectic bonding, gold-gold thermo-compression bonding or the like. Semiconductor devices are generally designed in such a form that connection electrodes formed on a silicon wafer (semiconductor device body or die chip) are exposed through an opening formed in a passivation film formed on the silicon wafer. Bump electrodes (projection electrodes) are formed on the connection electrodes. The connection electrodes are made of a base metal such as aluminum or aluminum alloy. In fabricating a semiconductor device with such a structure, an insulative natural oxide layer formed on the surface of each connection electrode can be removed before a bump electrode is formed on the connection electrode. A method similar to this is described in EP-A-0 316 912. The oxide removal process can be performed with argon ions by dry etching. To better protect the surface of a semiconductor device, the structure, in which a protective film made of polyimide is formed on the passivation film made of silicon nitride or the like, is proposed. In fabricating a semiconductor device with this structure, dry etching with argon ions is carried out to remove the natural oxide layer formed on the surface of the connection electrode, after the protective film made of polyimide is formed. The surface layer of the protective film is altered and deteriorated by the argon ions. This decreases the insulating resistance to result in inadequate insulation, so that electric leakage or short-circuiting may at last occur between the adjoining connection electrodes. JP-A-2246246 describes a provision of a connection electrode covered with an organic polyimide film. This organic film has an opening on the surface of the substrate, at least a part of the surface of the connection electrode being exposed through said opening. Remaining residue of the organic layer is removed with the help of a sputter cleaning process before an additional layer is applied. JP-A-58043540 and JP-A-5013585 describe the provision of projection electrodes projecting from the surface of the device. JP-A-61203654 and Journal of Vacuum Science and Technology A vol. 3, May 85, pgs. 772 to 776 describe multi-layer structures for electrically connecting a connecting electrode on top of a substrate. I EEE MCMC-1993, March 15, pgs. 74 to 78 describes the improvement of the adhesion strength of a conductive layer on top of a connection electrode by provision of additional metallic grains on top of the electrode. JP-A-4092432 describes a bumped electrode having polyimide and underlying silicon nitride passivation. It is the object of the present invention to provide an improved semiconductor device and an improved method of fabricating a semiconductor device which are not adversely effected by alteration of a surface layer of a protective film. This object can be achieved with a method of fabricating a semiconductor device having the features of the characterizing portion of claim 1. The present invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings. Figs. 1(A) to 1(E) are cross-sectional views illustrating each of the fabrication steps for a semiconductor device according to one embodiment of the present invention; Fig. 1(A) shows a structure in which a protective film, a layer for forming an intermediate connection film and a thin layer for forming a thin gold film are formed on a silicon wafer; Fig. 1(B) shows a situation in which exposure with a photomask is being performed after a photoresist film is formed on the silicon wafer; Fig. 1(C) shows a structure in which an opening is formed in the photoresist film on the silicon wafer; Fig. 1(D) shows a structure in which a bump electrode is formed in the opening formed in the photoresist film; and Fig. 1(E) shows a structure after the removal of the photoresist film and unnecessary portions of the layer for forming the intermediate connection film and the thin layer for forming the thin gold film. Fig. 2(A) is a plan view of a structure in which the bump electrodes of the semiconductor device are connected to finger leads of a TAB tape; Fig. 2(B) is a cross-sectional view of a part of the structure shown in Fig. 2(A); Fig. 3(A) shows an example of a structure useful in understanding of the invention; and Fig. 3(B) is a cross-sectional view showing a modification of the semiconductor device produced by the method of this invention. The semiconductor device produced according to one embodiment of the method of the present invention will now be described referring to Figs. 1(A) to 1(E). Figs. 1A to 1(E) are cross-sectional views illustrating the steps of fabricating a semiconductor device according to the embodiment. As shown in Fig. 1(A), an internal electrode 2, such as a gate electrode, and an insulating film made of silicon oxide or the like is formed on a silicon wafer (semiconductor substrate or semiconductor device body) 1. A connection electrode 4, made of a base metal such as aluminum or aluminum alloy, is formed on the insulating film 3. The connection electrode 4 is connected to the internal electrode 2 by an interconnection (not shown). A passivation film 5 of an inorganic material, such as silicon nitride, is formed on the entire surface of the insulating film 3 and the connection electrode 4. Then, an opening 6 is formed at a predetermined location in the passivation film 5 by etching, so that the center portion of the connection electrode 4 is exposed through this opening 6. The passivation film 5 around the connection electrode 4 and on the top of the insulating film 3 is left. A protective film 7 of an organic material, such as a resin like polyimide, is deposited about 1 to 5 µm thick on the entire surface of the passivation film 5 and the connection electrode 4. An opening 8 is formed at a predetermined location in the protective film 7 by etching, so that the center portion of the connection electrode 4 is exposed through the openings 6 and 8. The protective film 7 on the surface of the passivation film 5 excluding its opening 6 is left. In this embodiment, the opening 8 of the protective film 7 has a larger outer shape than the opening 6 of the passivation film 5. To remove an insulative natural oxide layer (not shown) formed on the surface of the connection electrode 4, dry etching with argon ions is performed on the entire surface of the silicon wafer 1 in vacuum. The surface layer of the protective film 7 made of polyimide is altered due to the argon ions, forming an alteration layer (not shown) with a thickness of about 100 to 200 nm. Next, to prevent the oxidization of the surface layer of the connection electrode 4, the silicon wafer 1 is subjected to the next step in vacuum. In the next step, an alloy, such as titan-tungsten, and gold are vapor-deposited or sputtered on the resultant structure in the named order, yielding a layer (alloy layer) 9 for forming an intermediate connection film and a thin layer (metal layer) 10 for forming a thin gold film with thicknesses of about, for example, 200 to 600 nm. Next, a photoresist drop is put on the surface of the thin film 10 and spin coating is then performed to form a relatively thick photoresist film 11 of about 20 to 30 µm thickness, as shown in Fig. 1(B). To make the photoresist film 11 relatively thick to have a thickness of about 20 to 30 µm, the photoresist drop used in this embodiment (e.g., BMR1000 produced by TOKYO OHKA KOGYO Co., Ltd.) should have a viscosity of 30 to 170 N·s/m 2 (300 to 1700 CPS (centipoises)), several 3 to 17 times greater than the viscosity of an ordinary one. The rotational speed for the spin coating should be 200 to 700 rpm. The photoresist film 11 is exposed using a predetermined photomask 12 as shown in Fig. 1(B). The exposed photoresist film 11 is developed to form an opening 13 at a predetermined location in the photoresist film 11. More specifically, the opening 13 is formed in the portion corresponding to the opening 8 of the protective film 7 and an area around the opening 8, as shown in Fig. 1(C). In this case, an organic solvent essentially consisting of xylene (e.g., C-3 produced by TOKYO OHKA KOGYO Co., Ltd.) is used as a developer. Then, gold is electroplated in the opening 13, forming a straight bump electrode (projection electrode) 14 on the top of the thin layer 10 in the opening 13, as shown in Fig. 1(D). To permit the bump electrode 14 to have a flatten top and a straight shape, the bump electrode 14 has a thickness of about 20 to 30 µm with its top surface not protruding from the top surface of the photoresist 11. After the formation of the bump electrode 14, the photoresist film 11 is removed using an organic solvent essentially consisting of ethyl cellosolve and dichlorobenzene (e.g., a release agent SP produced by TOKYO OHKA KOGYO Co., Ltd.). Next, with the bump electrode 14 used as an etching mask, the unnecessary portion of the thin layer 10 is etched out with an iodic etching solution so that a thin gold film 10a is formed by the remaining thin layer 10, as shown in Fig. 1(E). Next, with the bump electrode 14 used again as an etching mask, the unnecessary portion of the layer 9 is removed by dry etching, so that an intermediate connection film 9a is formed by the remaining layer 9. Then, dry etching is performed on the entire surface of the silicon wafer 1 by oxygen plasma to remove the alteration layer i.e., the surface layer of the protective film 7. A microwave ashing system may be used for this purpose. When a microwave ashing system MAS800 manufactured by CANON Inc. was used under the conditions of a frequency of 2450 MHz, output power of 500 W, a plate temperature of 150°C, an oxygen flow rate of 150 SCCM, pressure of 0.8 mmTorr and a processing time of 20 to 40 seconds, the surface layer of the protective film 14 made of polyimide was removed by an amount of about 200 to 500 nm. The surface of the bump electrode 14 of gold is not etched nor altered by the oxygen plasma. An RF (Radio Frequency) ashing system or the like may be used, or an ozone treatment or the like may be performed. According to the thus obtained semiconductor device, the surface layer of the protective film 7 is removed by dry etching after the bump electrode 14, the thin gold film 10a and the intermediate connection film 9a are formed. Therefore, even if the surface layer of the protective film 7 is altered by the dry etching to remove the insulative, natural oxide layer formed on the surface of the connection electrode 4, the adverse affect by the alteration of the surface layer of the protective film 7 can be prevented. With reference to Figs. 2(A) and 2(B), a description will now be given of the case where the bump electrodes of a semiconductor device fabricated in the above manner are bonded to the finger leads of a TAB tape. First, the silicon wafer 1 having undergone the above-described steps is diced into a plurality of semiconductor devices (chips) 21. A single semiconductor device 21 has a plurality of bump electrodes 14 mentioned above. A TAB tape 22 has a plurality of finger leads 23. The finger leads are formed by: patterning a copper foil 23b laminated on a base tape 24, into a predetermined shape; and then plating solder 23a on the surface of the portions of the patterns where the bump electrodes 14 of the semiconductor device 21 are to be connected. The finger leads 23 protrude into a device hole 25 formed in the base tape 24. The semiconductor device 21 is placed in the device hole 25, and the individual bump electrodes 14 are bonded to the associated finger leads 23 by soldering with the solder 23a. After the bump electrodes 14 of the semiconductor device 21 are connected to the finger leads 23 of the TAB tape 22, a protecting resin (not shown) is potted on the semiconductor device 21 to cover and protect the semiconductor device 21, and then the portion indicated by the dashed line in Fig. 2(A) is cut out. Figure 3 (A) shows an example of a semiconductor device which is useful in understanding the present invention. Contrary to Figure 1(E) the opening 8 of the protective film 7 has a larger outer shape than the connection electrode 4. Figure 3 (B) shows a modification of the semiconductor device produced by the method of the present invention. The opening 8 of the protective film 7 has a smaller outer shape than the connection electrode 4. In addition, the opening 8 of the protective film 7 has a smaller outer shape than the opening 6 of the passivation film 5. Although the bump electrode 14 is made of gold, in the above described examples, the bump electrode 14 may be made of solder.

Claims: Method of fabricating a semiconductor device, comprising the steps of: a) providing a semiconductor substrate (1) having a connection electrode (4) on one surface thereof; b) forming an organic film (7) over a periphery of said connection electrode (4) and the one surface of the semiconductor substrate (1), said organic film (7) having an opening (8) for exposing a middle portion of the connection electrode (4); c) dry etching the middle portion of said connection electrode (4), said middle portion being exposed from the opening of the organic film (7); d) covering said organic film (7) and said middle portion of said connection electrode (4) with a metal film (9,10), e) forming a bump electrode (14) on a portion of said metal film (9,10) which corresponds to said middle portion of said connection electrode (4) in such a manner that a peripheral portion of said organic film (7) near the opening (8) is located under said bump electrode (14); and f) removing the portion of said metal film (9,10) not covered by said bump electrode (14); characterized by the following step after step f): g) dry etching with oxygen the exposed portion of the organic film (7) . The method according to claim 1, characterized by further comprising the step of: forming an inorganic film (5) having an opening (6) formed on one surface of said substrate (1), at least a part of said surface of said connection electrode (4) being exposed through said opening (6). The method according to claim 2, characterized in that said opening (8) of said organic film (7) formed in said organic film forming step has a larger outer shape than said opening (6) of said inorganic film (5). The method according to claim 2, characterized in that said opening (8) of said organic film (7) formed in said organic film forming step has a smaller outer shape than said opening (6) of said inorganic film (5). The method according to one of the preceding claims, characterized in that dry etching is performed with argon in said step of etching said exposed surface layer of said connection electrode (4). The method according to any one of the preceding claims, characterized in that said organic film (7) is made of a resin in said organic film forming step. The method according to claim 6, characterized in that said resin includes polyimide. The method according to any one of the preceding claims, characterized in that said connection electrode (4) is made of a base metal in said connection electrode forming step.

Inventors: WAKABAYASHI TAKESHI, WAKABAYASHI, TAKESHI, WAKABAYASHI, TAKESHI, C/O CASIO COMPUTER CO., LTD.
Applicants: CASIO COMPUTER CO LTD, CASIO COMPUTER CO., LTD.
Application year: 1994
Main classification: H01L21
IPC codes: H01L23, H01L21